A contact image sensor (CIS) is one of linear sensors. The contact image sensor can scan a planar picture or document into an electronic file, which is then stored, displayed, processed or transmitted. Since the optical source, the lens and other components are integrated into a single module, the contact image sensor is light and slim. Moreover, since the contact image sensor is easily assembled, the contact image sensor is cost-effective.
FIG. 1 is a schematic view illustrating the architecture of a contact image sensor used in a contact-type image scanner according to the prior art. As shown in FIG. 1, the contact image sensor comprises a red LED 10, a green LED 11, a blue LED 12, a light-guiding member 13, a rod lens array 14 and a linear sensor array 15. The linear sensor array 15 is operated according to start pulses. During operation of the contact image sensor, the red LED 10, the green LED 11 and the blue LED 12 are successively turned on for each start pulse. During the on duration of each LED, the light emitted by the LED is guided to a be-scanned original 20 by the light-guiding member 13. The light is reflected by the original 20 and then focused on the linear sensor array 15 by the rod lens array 14. When the contact image sensor is linearly moved to transversely scan the original 20, the image information of three primary colors will be acquired.
FIG. 2 is a schematic timing waveform diagram illustrating the relations between the start pulses and the on durations of respective LEDs according to a prior art technology. In response to a first start pulse, the red LED 10 is turned on for an on duration Tr. Next, in response to a second start pulse, the green LED 11 is turned on for an on duration Tg. Next, in response to a third start pulse, the blue LED 12 is turned on for an on duration Tb. Since the contact image sensor is continuously and linearly moved to perform a scanning operation while sequentially turning on the light sources, a time lag is produced between the period of reading the red image (in response to the first start pulse) and the period of reading the green image (in response to the second start pulse). Similarly, two time lags are produced between the period of reading the red image (in response to the first start pulse) and the period of reading the blue image (in response to the third start pulse). Due to the time lag resulted from sequential turn-on operation while continuously moving the contact image sensor, a color misregistration problem occurs.
U.S. Pat. No. 6,545,777 disclosed an image reading apparatus with reduced color misregistration by adjusting the timing of turning on each LED. The relations between the start pulses and the on durations of respective LEDs are shown in FIG. 3. By controlling the end of the red LED on duration Tr to be synchronous with the rising edge of the second pulse, the color misregistration for each scan line is slightly reduced when comparison with FIG. 2. That is, the overall time interval b is slightly shorter than the overall time interval a. Since the scanning speed is gradually increased, the color misregistration is still unsatisfied.
U.S. Pat. No. 7,535,606 disclosed a RYB (red-luminance-blue) sampling method of a contact-type image scanner. The relations between the start pulses and the on durations of respective LEDs are shown in FIG. 4. The equations mapping RGB to YCbCr can be rearranged to yield Cr and Cb as the functions: Y=0.29900×R+0.58700×G+0.11400×B, Cb=−0.16874×R−0.33126×G+0.50000×B=0.56433×(B−Y); and Cr=0.50000×R−0.41869×G−0.08131×B=0.71327×(R−Y). As shown in FIG. 4, the red LED is turned on for an on duration Tr in response to the first start pulse, and the blue LED is turned on for an on duration Tb in response to the third start pulse. In response to the second start pulse, the red, green and blue LEDs are sequentially turned on, wherein the red LED on duration Ty,r=0.299×Tr, the green LED on duration Ty,b=0.587×Tg, and the blue LED on duration Ty,b=0.114×Tb. As such, a Y value is acquired. After the RYB value is acquired, the Y, Cr and Cb can be deduced according to the following equations: Cr=0.71327×(R−Y) and Cb=0.56433×(B−Y). In practice, the color misregistration for each scan line is about a three-pulse length. The time lag resulted from sequential illumination of the red, green and blue LEDs produces a scanning position error of the three primary colors.
Therefore, there is a need of providing a contact-type image scanner and a scan controlling method so as to obviate the drawbacks encountered from the prior art.